Multimode fibers have constantly evolved from the very beginning of optical communications industry through the recent and on-going explosion of the Ethernet traffic. Enabled by VCSEL technology, high-speed multimode optical fibers, such as OM4 fibers (which are laser-optimized, high bandwidth 50 μm multimode fibers, standardized by the International Standardization Organization in document ISO/IEC 11801, as well as in TIA/EIA 492AAAD standard), have proved to be the medium of choice for high data rate communications, delivering reliable and cost-effective 10 to 100 Gbps solutions. The combination of Wide-Band (WB) multimode fibers with longer-wavelengths VCSELs for Coarse Wavelength Division Multiplexing (CWDM) is an interesting option to be considered in order to meet the future increase of demand. By wide-band multimode fiber, it is meant here, and throughout this document, a multimode fiber having an operational wavelength range larger than 20 nm, for example an operational wavelength range comprised between 850 nm and 950 nm or beyond.
However, the high modal bandwidth of OM4 fibers has until now only been achieved over a narrow wavelength range (typically 850 nm+/−2 nm, or 850 nm+/−10 nm). The feasibility of Wide-Band (WB) multimode fibers satisfying OM4 performance requirements over a broader wavelength range is a challenge to overcome for next generation multimode systems.
The OM4 fiber performance is usually defined by an Effective Modal Bandwidth (EMB) assessment at a given single wavelength. For instance, OM4 fibers should exhibit EMB larger than 4,700 MHz-km at a wavelength of 850 nm. The achievement of such high EMB values requires an extremely accurate control of refractive index profile of multimode fibers. Up to now, traditional manufacturing process cannot guarantee so high EMB, and generally it is hard to accurately predict the EMB values from refractive index profile measurements on core rod or cane, especially when high EMB (typically larger than 2,000 MHz-km) are expected, meaning the fiber refractive index profile is close to the optimal profile. As a matter of fact, EMB are directly assessed on fibers.
In order to minimize modal dispersion, the OM4 fibers generally comprise a core showing a refractive index that decreases progressively going from the center of the fiber to its junction with a cladding. In general, the index profile is given by a relationship known as the “α profile”, as follows:
            n      ⁡              (        r        )              =                            n          0                ⁢                              1            -                          2              ⁢                                                          ⁢                                                Δ                  ⁡                                      (                                          r                      a                                        )                                                  α                                                    ⁢                                  ⁢        for        ⁢                                  ⁢        r            ≤      a        ,where:
n0 is a refractive index on an optical axis of a fiber;
r is a distance from the optical axis;
a is a radius of the core of the fiber;
Δ is a non-dimensional parameter, indicative of an index difference between the core and a cladding of the fiber; and
α is a non-dimensional parameter, indicative of the general shape of the index profile.
When a light signal propagates in such a core having a graded index, the different modes experience a different propagation medium, which affects their speed of propagation differently. By adjusting the value of the parameter α, it is thus possible to theoretically obtain a group velocity, which is virtually equal for all the modes and thus a reduced intermodal dispersion for a particular wavelength.
Hence, the Alpha parameter (α) that governs the shape of this graded-index core can be tuned to maximize the modal bandwidth at 850 nm of OM4 multimode fiber, the typical operating wavelength of high-speed data communications. A given alpha parameter value is generally selected to offer an optimum EMB as illustrated in document “WideBand OM4 Multi-Mode Fiber for Next-Generation 400 Gbps Data Communications” by Molin et al. ECOC 2014.
The Effective Modal Bandwidth (EMB) is assessed by a measurement of the delays due to the modal dispersion, known under the acronym DMD for “Dispersion Modal Delay”. The DMD measurement consists in recording pulse responses of the multimode fiber for single-mode launches that radially scan the core. It provides an accurate cartography of the modal dispersion of the multimode fiber, called the DMD plot, that is then post-processed in order to assess the minimal EMB a fiber can deliver at a given wavelength. The DMD measurement procedure has been the subject of standardization (IEC 60793-1-49 and FOTP-220) and is also specified in Telecommunications Industry Association Document no. TIA-455-220-A. Each DMD metric, or DMD value, is expressed in units of picoseconds per meter (ps/m) so that the total delay is normalized by fiber length. It determines the delay between the fastest and the slowest pulses traversing the fiber considering a collection of offset launches normalized by fiber length. It basically assesses a modal dispersion. Low DMD value, i.e. low modal dispersion as measured by DMD, generally results in higher EMB.
Basically, a DMD graphical representation is obtained by injecting a light pulse having a given wavelength at the center of the fiber and by measuring the pulse delay after a given fiber length L, the introduction of the light pulse of a given wavelength being radially offset to cover the entire core of the multimode fiber. Individual measurements are thus repeated at different radial offset values so as to provide cartography of the modal dispersion of the examined multimode fiber. The results of these DMD measurements are then post-processed to determine an effective transfer function of the optical fiber, from which a value of EMB may be determined.
Nowadays, all multimode fiber manufacturers perform DMD measurements and EMB assessment, at a single wavelength only, of their whole production: typically at 850 nm+/−2 nm for OM4 qualification and at 850 nm+/−10 nm for OM3 qualification.
With the advent of new multimode fiber applications, requiring high EMB over a wide operating window, one of the main concerns of the multimode fiber manufacturers is to have the ability to easily assess the EMB over a wide wavelength range, for example between 850 nm and 950 nm or beyond.
Using the aforesaid classical measurement procedure (comprising a series of DMD measurements and an EMB assessment at a single wavelength) to assess the optical fiber's EMB over a range of wavelengths, i.e. at a plurality of wavelengths, would require performing several measurement procedures at said wavelengths adequately spread over the wavelength range of interest. However, making distinct independent DMD measurements to qualify the optical fiber's EMB at multiple wavelengths shows several drawbacks:                first, it would imply building new test beds at the manufacturer's plant, each test bed being dedicated to a light source emitting at a given wavelength. This would represent a complex and costly operation.        Then, it would greatly increase measurement time of the manufactured multimode fibers. Keeping on measuring the whole production of multimode fibers would thus greatly increase the production cost of the fibers. Alternately, manufacturers could decide to measure only part of their production, in order to maintain production costs at an adequate level, but this would lead to a decrease in the quality of the sold fibers.        
An interesting option would be to limit these measurements to fibers that are the most likely to fulfill such a wide band EMB requirement. Actually, it would allow reducing the wasted measurement time spent on measuring low bandwidth fibers.
To this purpose, in “Expansion of the EMBc Calculation to a Complete Fiber Bandwidth Characterization”, Proc. 58th Internat. Wire & Cable Sumposium (IWCS'09), Charlotte, N.C., USA, 2009, Andreas Huth and Harald Hein disclose a technique for predicting the overfilled launch bandwidth (OFLBW) of fibers at 1300 nm using only DMD measurements at 850 nm. Such a technique relies on a transformation of the DMD plot. Actually, the authors have observed a relation between the DMD measurement results of a fiber at different wavelengths, and have derived from these observations a transformation function, in the form of a shift, allowing them to predict the DMD plot of a fiber at 1300 nm, knowing the DMD plot of the fiber at 850 nm.